Every time there’s an earthquake in the Midwest, my mother emails me, just in case I want to move back home to study it. So that’s how I heard about yesterday morning’s earthquake in Illinois – a bit less exciting than waking up to it, but that’s fine with me.
This is not earthquake enough to reverse the geoscientific brain drain from the Midwest to the West Coast (there’s also the weather to think about)… but for an ostensibly stable part of the continental interior, thousands of miles from the nearest plate boundary, Illinois and Indiana have seen an awful lot of seismic action over the past 12,000 years:
Estimated locations of prehistoric earthquakes in Illinois and Indiana, from Obermeier (1998). Stars are historical earthquakes (1804-1992) of magnitude 5 or greater, dots are magnitude 4.5-5, and the light gray plus signs are weer than 4.5. The prehistoric earthquakes on this map were found by digging around stream banks, looking for preserved sand boils and other evidence of liquefaction.
At the bottom of this map are the famous New Madrid earthquakes of 1811 and 1812, which made the Mississippi run backward and rang church bells in Boston. Are they connected to yesterday morning’s event?
The short answer is yes and the long answer is sort of, but it’s not as simple as you are probably imagining.
Six hundred million years ago, the former supercontinent of Rodinia was in the late stages of a nasty breakup. Laurentia (the tectonic unit now known as North America) actually tried to split apart in several places, but somehow managed to hold itself together. But the experience left scars.
We are fairly certain that one of the scars from that breakup lies underneath the Mississippi River near the town of New Madrid, Missouri. Six hundred million years ago the area would have looked much like the East African Rift Valley does today, like it’s almost ready to give birth to a brand new ocean.
These failed rifts are weak spots. Like an old injury when bad weather approaches, they will pop and crackle whenever the tectonic weather is favorable.
Four arms of the failed rift zone? From Nelson 1990, who modified it from Braile et al. 1982.
So that’s New Madrid. But yesterday’s earthquake wasn’t in the New Madrid Seismic Zone, it was in the Wabash Valley Seismic Zone. The WVSZ is roughly coincident with the “Southern Indiana arm” drawn in dotted lines on the above figure. Those lines were drawn based on gravity and magnetic data (there is something heavy and made of iron down there, which might be basalt left over from the rift). The WVSZ also contains faults that are approximately the same age as the failed rift. However, there’s not much structural or seismic evidence to support the idea that they’re one and the same rift valley.
So, the earthquake in Illinois yesterday was probably related to the New Madrid earthquakes, inasmuch as both events occurred on weak spots left over from a particularly violent period in North America’s tectonic history. But they’re not really part of the same structure, and they’re certainly not on the same fault.
Today’s event was probably too small to have affected the chances of a big quake at New Madrid one way or the other.
- S.F. Obermeier (1998) Liquefaction evidence for strong earthquakes of Holocene and latest Pleistocene ages in the states of Indiana and Illinois, USA. Engineering Geology 50 227-254, doi:10.1016/10.1016/S0013-7952(98)00032-5
- W. John Nelson (1990) Comment on “Major Proterozoic basement features of the eastern midcontinent of North America revealed by recent COCORP profiling” Geology 18 378. (See also Pratt et al., Reply, and the original 1989 article by Pratt et al.)
- Won-Young Kim (2003), The 18 June 2002 Caborn, Indiana, Earthquake: Reactivation of Ancient Rift in the Wabash Valley Seismic Zone? Bull. Seis. Soc. Am. 93, 2201-2211, doi:10.1785/0120020223